Add isWithin method to Date extensions

[kaphacius]

another go on #258

Checklist

• I checked the Contributing Guidelines before creating this request.
• New extensions are written in Swift 4.
• New extensions support iOS 8.0+ / tvOS 9.0+ / macOS 10.10+ / watchOS 2.0+.
• I have added tests for new extensions, and they passed.
• All extensions have a clear comments explaining their functionality, all parameters and return type in English.
• All extensions are declared as public.

[SwifterSwiftBot]

	2 Warnings
⚠️	Consider adding tests for new extensions or updating existing tests for a modified SwifterSwift extension
⚠️	The source files have been modified. Please consider adding a CHANGELOG entry if necessary.

	1 Message
📖	Thank you for submitting a pull request to SwifterSwift. The team will review your submission as soon as possible.

Generated by 🚫 Danger

[codecov]

Codecov Report

Merging #288 into master will decrease coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #288      +/-   ##
==========================================
- Coverage   94.51%   94.43%   -0.08%     
==========================================
  Files          99       99              
  Lines        5503     5520      +17     
==========================================
+ Hits         5201     5213      +12     
- Misses        302      307       +5

Flag	Coverage Δ
#ios	94.23% <100%> (-0.08%)	⬇️
#osx	60.85% <100%> (+0.03%)	⬆️
#tvos	89.16% <100%> (-0.06%)	⬇️

Impacted Files	Coverage Δ
Tests/FoundationTests/DateExtensionsTests.swift	94.89% <100%> (+0.1%)	⬆️
Sources/Extensions/Foundation/DateExtensions.swift	98.31% <100%> (-1.69%)	⬇️

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[kaphacius]

what happened? how did you push into my master? :)
not sure why the build fails now

[omaralbeik]

@kaphacius Sorry It looks like your #289 PR, had a conflict with this PR, I fixed it manually from here which added a commit to your master.
Could you please check and open the request again